The Nobel Prize is an award that honors pioneers. In his last will and testament, the prize’s founder Alfred Nobel stated that the award was to be given to persons who had made the most important discovery, invention, or improvement in certain fields. The words ‘most important’ imply that the recipient was a pioneer in their field and that their achievement generated momentum for that line of research. In fact, we in the general public are familiar with the momentum in research fields stemming from the achievements of pioneers such as Einstein, who fundamentally changed our perception of physics since Newton, and Watson and Crick, who discovered the double helix structure of DNA, the keystone of modern molecular biology. In recent years, we have also become more aware of the impact of these trends on society. If the iPS cells created by Dr. Shinya Yamanaka, winner of the 2012 Nobel Prize in Physiology or Medicine, gain widespread use in the future, we will be forced to change our perceptions of life and longevity.
Dr. Syukuro Manabe, Senior Meteorologist at Princeton University in the United States, who was awarded the Nobel Prize in Physics in 2021, is truly a pioneer in climate research through the development of climate models and computer simulations using these models. In terms of social impact, he created the prototype numerical model that establishes the scientific basis for understanding the issue of major concern that is global warming.
But what was this numerical model created by the pioneering Dr. Syukuro Manabe? Let’s explore Dr. Manabe’s research and subsequent developments, inviting commentary from three researchers with a close professional interest in Dr. Manabe’s research field: Professor Kaoru Sato, Professor Yukio Masumoto, and Associate Professor Hiroaki Miura.
Dr. Manabe’s First Achievement, the Remarkable One-dimensional Radiative-Convective Equilibrium Model
To understand Dr. Manabe’s research, we must first understand the greenhouse effect on Earth.
Sato: “Sunlight (solar radiation) shines down on the Earth every day, and the Earth also emits the same amount of infrared radiation upward, so as to maintain the energy balance. If the ground kept only receiving solar radiation continuously, the temperature of the Earth would simply rise and rise, if the Earth had no atmosphere, the ground would emit the infrared radiation from the Earth back into space. Solving the equation for equal amounts of radiation from both the Sun and the ground shows that the temperature of the ground should be 255 Kelvins (K; minus 18°C).”
However, the Earth does have an atmosphere, one that contains water vapor and carbon dioxide. This atmosphere has the property of transmitting most of the solar radiation, but absorbing most of the infrared radiation from the ground. The atmosphere radiates half as much infrared radiation upward and half downward as it receives from the ground. Thus, the ground receives two types of radiation: from the Sun and from the atmosphere. Naturally, the ground must emit a great deal of radiation back to avoid becoming too warm (radiation emitted from the ground is proportional to the fourth power of temperature).
The temperature of the ground now works out to be 290 K (about 17°C). Since 290 K minus 255 K equals 35 K, this means that the greenhouse effect of the earth’s atmosphere is 35 K.
Sato: “If the earth had no atmosphere, the temperature on its surface would be minus 18°C, which is very cold, but the greenhouse effect makes the temperature comfortable for humanity.”
What is the vertical temperature distribution in the atmosphere? If we divide the atmosphere into several horizontal layers and calculate the temperature distribution from the incoming radiation, we find that the layer nearest the ground is hotter and the upper layers rapidly become cooler. This is a similar situation to a pot of water being heated from below on a stove. But naturally, convection also occurs in the atmosphere.
In 1964, Dr. Manabe published his first remarkable achievement, a one-dimensional radiative-convective equilibrium model. Computers at the time could not directly calculate the convective flows that stir the atmosphere up and down, but Dr. Manabe boldly hypothesized that convection regulates the temperature so that it decreases by 6.5 Kelvin (K)/km with increasing altitude, thereby incorporating convection into his numerical model.
Sato: “The interesting thing about Dr. Manabe’s numerical model of the atmosphere is that it yields solutions not only for the troposphere, but also for the stratosphere above it. At the time, the Concorde airliner, which was designed to cruise at supersonic speeds in the stratosphere, was being developed, so they also solved the equation for the stratosphere to consider its impact on the atmosphere.
Dr. Manabe added trace components of the atmosphere — water vapor, carbon dioxide, and ozone —to this numerical model. These three components were then subtracted one by one and the calculation repeated to ascertain the effects on atmospheric temperature distributions. For example, if he removed ozone from the simulation, the temperature in the troposphere did not change greatly, but decreased sharply in the stratosphere. This is because of the ozone layer in the stratosphere that absorbs ultraviolet radiation and warms the atmosphere. This process of subtraction and recalculation showed that of the 35 K greenhouse effect, 10 K was due to the presence of carbon dioxide.
Dr. Manabe also calculated what would happen if the amount of carbon dioxide in the atmosphere doubled, finding that the surface temperature would increase by 2.3°C.
Sato: “This is almost the same as the predictions of the current numerical models in the Intergovernmental Panel on Climate Change (IPCC) reports and other publications. The fact that this number was published way back in 1964 is amazing. Today, the topic of global warming is one of worldwide concern, and that Dr. Manabe had the foresight to ask this question at a time when such a future could not even be imagined, and to come up with his prediction is a truly impressive achievement.
An Evolving Coupled Atmosphere-Ocean Model
Dr. Manabe’s second major achievement was the development of a coupled atmosphere-ocean model. At the time, he was working at the U.S. Geophysical Fluid Dynamics Laboratory (GFDL), and one of his colleagues was Dr. Kirk Bryan, an oceanographer who was building numerical models of the ocean. Dr. Joseph Smagorinsky, the then GFDL director, suggested that their research be combined into a numerical model, and the two colleagues completed a coupled atmosphere-ocean model in 1969. In this, they combined the atmosphere with the ocean to create a climate simulation model to cover the entire planet.
Dr. Manabe’s unique creative skills were also utilized in the development of this model. A numerical model covering the entire globe would require an enormous number of computations. Computers in the 1960s did not have the power to handle such a task, so what did Dr. Manabe and Dr. Bryan do?
Masumoto: “They cut out a strip of the earth that contained oceanic and continental portions and simulated what would happen within that strip. (Figure 2.) They found that the conditions in the atmosphere and interior of the oceans obtained from observations could be accurately reproduced in the numerical model. So now they were able to reproduce climate variability and climate change in a model in which the atmosphere and the oceans are coupled together.”
In 1991, Dr. Manabe used this coupled atmosphere-ocean model to simulate how the climate would change if the atmospheric carbon dioxide concentration doubled. The model showed that surface air temperatures would rise considerably near the Arctic region and not so much near the South Ocean. Comparison of these results with actual observational data that we continue to obtain to the present day shows that they are very similar to the changes that have occurred in reality. It means that already in the 1990s they were predicting the current level of global warming quite accurately.
The coupled atmosphere-ocean model created by Dr. Manabe has since been progressively improved by researchers who have added other complex physical, chemical, and biological processes that contribute to climate change. These include chemical changes in the atmosphere and oceans, changes in sea ice and glaciers, and respiration (taking in oxygen and releasing carbon dioxide) and photosynthesis (taking in carbon dioxide and releasing oxygen) processes by plants on land. The numerical model has since been developed into more sophisticated models known as Earth System Models by incorporating various realities of the Earth. However, the framework of the model was all Dr. Manabe’s. The numerical model used in the IPCC report that has continued to sound the alarm about global warming (the sixth report has now been released) is also based on Dr. Manabe’s model. As a pioneer, Dr. Manabe’s research has generated a major trend and has had a profound impact on society.
A Researcher’s Eyes, Looking Decades Ahead
Let’s now ask our three panelists what they think is so special about Dr. Manabe.
Miura: “In the 1940s, the mathematician and information scientist Dr. John von Neumann teamed up with the meteorologist Dr. Jule Charne to study numerical weather prediction as part of an electronic computer project. So numerical weather forecasts were available before Dr. Manabe. The Japan Meteorological Agency was also engaged in advanced research and began numerical weather forecasting in 1959 with the introduction of the IBM 704 computer. But Dr. Manabe was aiming for something much bigger than weather forecasting”
Sato: “Dr. Manabe was thinking seriously about the climate of the entire planet rather than weather, and on a long time scale of tens or hundreds of years. The sheer size of the subject and scale of time for making predictions were much greater.”
Miura: “What I thought was wonderful was that at a time when the processing power of computers was incomparably lower than that of today’s supercomputers, he made the bold simplifying hypothesis I mentioned earlier of representing the effect of convection as a rate of decrease in temperature of 6.5 K per kilometer of altitude.”
Sato: “Calculations using a one-dimensional radiative-convective equilibrium model show that stratospheric temperatures in the polar regions in winter are tens of degrees cooler than actual observed data. Dr. Manabe wrote in his paper that this ‘probably must have something to do with atmospheric motion,’ and that was the most impressive thing for me. This problem of the predicted low stratospheric temperatures in the polar regions is actually yet to be solved. Current climate models don’t yield temperatures that are tens of degrees lower, but even so, less precise models yield temperatures of about 20 degrees lower. This is called stratospheric cold bias, and it is still the subject of debate. Amazingly, Dr. Manabe had already identified it in the 1960s.”
Masumoto: “The method described by Dr. Sato earlier of comparing results with and without ozone is an insightful method that continues to be valid today. You can test a numerical model under conditions that are different from those on Earth, one of the techniques known as numerical experimentation in the sense that it is an experiment using a numerical model. So we can draw a straight line from Dr. Manabe’s research practice to the methods we use today.”
Sato: “One more thing. Dr. Manabe is also well aware of the limitations of models and often says that we should not rely too much on complex models. You’re always very aware that the numerical model you have developed is just that, a model.”
From a professional researcher’s point of view, does Dr. Manabe’s research make such an impression? It’s not easy for the average person to understand.
Other Researchers Build on the Foundations Established by Pioneers
Our three panelists noted that their research projects are extensions of Dr. Manabe’s work. What kind of research are they actually doing?
Masumoto: “I’m studying interactions between the atmosphere and ocean in the tropics, mainly using numerical models. While Dr. Manabe’s research is mostly on long time scales, such as hundreds of years in the future or even longer, mine deals with short time scale variations on the order of a few years, such as we see with the El Niño phenomenon. My research is similar to the work by Dr. Kikuro Miyakoda (now deceased), who was at GFDL with Dr. Manabe and mainly worked on short time scales. But I was also influenced by Dr. Manabe in the sense that we use similar numerical models.”
Miura: “I was strongly influenced by UCLA’s Dr. Akio Arakawa (now deceased), and I’m studying clouds to answer the question of why clouds organize the way they do. The framework of Dr. Manabe’s one-dimensional radiative-convective equilibrium model is still in use today and has evolved into the simulation of radiative-convective equilibrium systems representing three-dimensional flow and clouds that I’m working on. I’m also working on a research project on climate simulation using a global cloud-resolving model, a topic recommended to me by my academic advisor, Dr. Masahide Kimoto.
Sato: “I’m not engaged in research to create models per se, but I am working rather on more theoretical research. Dr. Manabe focused mainly on atmospheric layers up to the stratosphere, where the ozone layer is located, and I’m studying the climate and weather in the stratosphere and the atmosphere up to about 100 km above it, called the mesosphere and lower thermosphere, which are well mixed due to circulation, wave motion, and turbulence. I think you may be familiar with a cloud called a lenticular cloud that occasionally forms above Mount Fuji. Such clouds are what we call atmospheric gravity waves. When the wind blows, air is lifted and vertically displaced upward as a wave. We have found that these atmospheric gravity waves play a role in warming the cold stratosphere. The cold bias I mentioned earlier may be caused by the expression of an atmospheric gravity wave. In addition to simulating atmospheric gravity waves with a numerical model, we are also collecting actual observations. We observe the Antarctic sky using a large atmospheric radar system (a phased-array radar with 1,000 regularly arranged 3-meter-high antennas). I myself went to Syowa Station in Antarctica in 2018.”
Each of you is involved in Dr. Manabe’s numerical model in your own way. Simulation with numerical models seems to be indispensable for the study of science. How will computer simulations be used in scientific research in the future?
Miura: “Over the history of the scientific method, we see an evolution from experimental science to theoretical science and then simulation science. About 30 years ago, there was still an air of disbelief in computer simulations. However, it is now established as a fundamental technology. I think it will continue to increase in importance, with numerical models being recognized as wrong when they are mathematically and physically wrong, and then corrected.”
Masumoto: “Simulation allows us to study phenomena that can’t be investigated experimentally. I think it will go forward as a tool that’s inseparable from science.”
Sato: “In terms of using computers, I believe that deep learning and AI, as well as simulation, will find increasing value in future meteorological and oceanic research. Big data may enable more accurate predictions and simulation promises to be a very powerful tool.”
Broad-ranging Curiosity and Thinking for Oneself
Finally, what message would you like to give to the young people reading this article who are interested in the field of science?
Masumoto: “Be interested in a variety of fields, not just the one you want to pursue. For example, we now live in an era in which the study of weather and climate involves not only physics but also chemistry and biology. If you want to become a researcher, you should prepare a foundation that allows you to develop a wide range of research interests.”
Sato: “As Dr. Manabe said, a sense of curiosity is all-important. Always keep asking the question ‘Why?’. And you should learn to think for yourself. Also, I hope you will be interested in earth science. Explaining earth science in high school is limited in terms of the physical laws that can be used, which makes it rather difficult to understand, but if you study it in college, you’ll find that it is actually a fascinating field.”
Miura: “I hope that this form of scientific inquiry will continue to flourish: describing the world we live in using equations and numerical models. However, setting aside what those old people are doing (laughs), I want young people to think carefully about how they want to live their lives and resolutely choose their own path. Whether you choose to study science, engineering, economics, or even not to go to college at all, think carefully about your choices. If, after careful consideration, you develop a strong interest in what we are doing, then the School of Science is an excellent option. We’d love to have you join us.”
Listening to the explanations of Dr. Manabe’s research and that being conducted by our three panelists, we can see the academic landscape in which the remarkable research achievements of pioneers are inherited and further developed. The numerical models established by the pioneers will continue to evolve more widely, deeply, and with greater precision. If you enter into this world, you will certainly sense the footsteps of many pioneers.
※Year of interview:2022
Interview/Text: Osamu Shimizu [ ACADEMIC GROOVE ]
Photography: Junichi Kaizuka